viewerGL: load images with tuttle if available

* improve load_texture function
* use image bounds, and not only image size
* no more clearcolor
viewerGL: split code between load_image and GLViewport

If tuttle not detected still use PIL
viewerGL: allows to load images with TuttleOFX
Merge branch 'develop' of github.com:buttleofx/ButtleOFX into develop